Giant asteroid Vesta is so enormous, many astronomers don't even refer to it as an asteroid, preferring to call the celestial body a "protoplanet." This image, which compares Vesta to eight other asteroids that we've sent spacecraft to investigate, helps illustrate the reason such a distinction exists.

A hi-res version of the image up top (one where you can actually see 25143 Itokawa) is available here.

At roughly 330 miles in diameter, Vesta is not only the largest asteroid we've ever visited (NASA's Dawn spacecraft has been orbiting Vesta since July), it's nearly the largest asteroid in our entire Solar System. Second in size only to Ceres, the two bodies are estimated to account for over 40% of the mass in the entire main asteroid belt.

Dawn, which recently maneuvered into its closest orbit around Vesta, is scheduled to continue investigating the protoplanet until the middle of 2012, when it will set course for Ceres. By investigating these massive celestial bodies, scientists hope to acquire a better understanding of what conditions were like during our Solar System's infancy. [NASA | Spotted on PBS]